1. Field of the Invention
The present invention relates to a toner used in electrophotography, electrostatic recording, magnetic recording and the like.
2. Description of the Related Art
A number of methods are conventionally known as methods for electrophotography. In general, copies or prints are obtained by forming an electrostatic latent image on an electrostatic latent image bearing member (hereinafter also termed “photosensitive member”) by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a toner image as a visible image, transferring the toner image to a recording medium such as paper as occasion calls, and thereafter fixing the toner image onto the recording medium by the action of heat and/or pressure. Apparatus for such image formation include copying machines, printers and so forth.
These printers or copying machines are being changed over from analogue machines to digital machines, and it is strongly sought to have a good reproducibility of latent images and a high resolution and at the same time to achieve higher image quality and energy saving.
Various approaches are taken to the achievement of higher image quality. In particular, making dot reproducibility higher and making any density non-uniformity not come about at the time of fixing are given as important items. Meanwhile, about a recording medium paper, the paper is becoming rich in variety, including wood free paper, which is small in any surface unevenness (hills and dales) coming from fibers, and what is called antique paper (paper with rough surface), which is large in such surface unevenness.
Here think about the density non-uniformity at the time of fixing. Density non-uniformity due to a difference in density or gloss tends to come about in the case of the antique paper, which is large in the surface unevenness coming from fibers. This is because, at hills (areas of surface that are higher than the areas around them) of paper surface that come from fibers, heat and pressure are readily applied to the toner that is present there at the time of fixing and hence the toner is sufficiently pressed and spread thereon to become fixable to the paper. If, however, the heat and pressure are applied thereto in excess, the toner may so much soak into fibers of the paper as to result in low gloss or density. Such a phenomenon may be seen. On the other hand, at dales (areas of surface that are lower than the areas around them) of paper surface that come from fibers, sufficient heat and pressure can not readily be applied to the toner, and hence the toner tends to be insufficiently melted there to tend to result in low density or gloss.
Moreover, where any fixing assembly of printers or copying machines is made simple or fixing temperature is set lower for the purpose of energy saving, the specific heat a sheet of paper has at its leading end area and rear end area from the fixing assembly tends to greatly differ between them, and hence the density non-uniformity more tend to come about. Also, the density non-uniformity may remarkably come about in halftone images, having a small toner laid-on level on the paper.
More specifically, in order to make any density non-uniformity not easily come about at the time of fixing, it is a subject for the toner to have a uniform fixing performance over broad pressure and temperature ranges.
For this subject, various improvements have been attempted in terms of toners. In Japanese Patent Application Laid-open No. 2003-280270, a toner is proposed which is a toner containing at least a binder resin and a colorant, wherein the binder resin component is a polyester resin, contains from 5% by mass or more to 30% by mass or less of THF-soluble matter, and also has a peak (P1) in the elution volume range of absolute molecular weight of from 3.0×103 or more to 3.0×104 or less, a peak (P2) in the elution volume range of absolute molecular weight of from 5.0×104 or more to 6.0×105 or less and a peak (P3) in the elution volume range of absolute molecular weight of from 2.0×106 or more to 5.0×107 or less, each in the relationship between elution volume and light scattering intensity which are obtained by a light scattering detector in GPC-MALLS analysis of the THF-soluble matter.
In Japanese Patent Application Laid-open No. H11-160909, also proposed are a process for producing a polymerization toner which process comprises the step of subjecting a polymerizable monomer composition to suspension polymerization in the presence of an oil-soluble polymerization initiator until polymerization conversion comes to be within the range of from 30% or more to 97% or less, and the step of adding a water-soluble polymerization initiator into an aqueous dispersion medium while the polymerization conversion is within the above range, to further continue the suspension polymerization to form colored polymer particles, and also a process for producing a polymerization polymer of core/shell structure which process comprises the step of further subjecting a polymerizable monomer for shells to suspension polymerization in the presence of the colored polymer particles to form colored polymer particles on the surfaces of which polymer layers serving as shells have been formed.
In Japanese Patent Application Laid-open No. 2006-343372, also proposed is a process for producing a toner by polymerizing in an aqueous medium a polymerizable monomer composition containing at least a polymerizable monomer and a colorant, wherein a peroxydicarbonate compound is used as a polymerization initiator.
In all these toners, their low-temperature fixing performance, developing performance and storage stability are improved. However, there has still been room for further improvement in respect of their low-temperature fixing performance and dot reproducibility. Further, the effect against the density non-uniformity has been insufficient, and there has been room for improvement.